[Mister Underhill]

Originally Posted by alatar

...what chance would someone, even Gandalf, have against one of the Fellowship who took the Ring to do something with it?

This is an excellent question, and one whose answer is not as clear-cut as some speculation on this thread would lead you to believe. Letter 246, in which Tolkien expounds at length on the events at the Crack and possible alternative outcomes, is particularly relevant to the question of the Ring's power or lack thereof. Like so many topics in Tolkien, it offers no easy, pat answers, though something caught my eye which suggests that the Ring does have power which would be instantly accessible to a claimant.

The situation as between Frodo with the Ring and the Eight might be compared to that of a small brave man armed with a devastating weapon, faced by eight savage warriors of great strength and agility armed with poisoned blades. The man's weakness was that he did not know how to use his weapon yet; and he was by temperament and training averse to violence. Their weakness that the man's weapon was a thing that filled them with fear as an object of terror in their religious cult, by which they had been conditioned to treat one who wielded it with servility.

Tolkien, Master of Ambiguity, suggests two possible readings here, I suppose: on the one hand, he explicitly attributes great offensive power to the Ring (supposing you know how to use it); then a moment later he relates the Ring's power to that of an icon -- possibly more powerful as an idea than actually powerful.

But I'm also quoting a little bit out of order. A few paragraphs back, Tolkien seems to suggest beyond a shadow of a doubt that the Ring has some power to offer even an unskilled user, supposing that he uses it in the 'proper' spirit, i.e., aggressively:

It is an interesting problem: how Sauron would have acted or the claimant have resisted. Sauron sent at once the Ringwraiths. They were naturally fully instructed, and in no way deceived as to the real lordship of the Ring. The wearer would not be invisible to them, but the reverse; and the more vulnerable to their weapons. But the situation was now different to that under Weathertop, where Frodo acted merely in fear and wished only to use (in vain) the Ring's subsidiary power of conferring invisibility. He had grown since then. Would they have been immune from its power if he claimed it as an instrument of command and domination? Not wholly. I do not think they could have attacked him with violence, nor laid hold upon him or taken him captive...

alatar is also right to bring up the Ring's power of invisibility as a factor which, all by itself, could cause a lot of difficulty for someone trying to take the Ring from a claimant.